Group III-V nitride-based semiconductor light emitting devices are used as a light source for various displays and illumination apparatuses. Use as a light source for indoor optical communication is also investigated. In applying group III-V nitride-based semiconductor light emitting devices to these uses, enhancement of light emission efficiency is a problem.
For enhancing the light emission efficiency of a light emitting device, required are (1) suppression of nonradiative center by a device structure enhancing recombination rate of electrons and holes or by reduction of dislocation and defect densities of a semiconductor crystal (improvement of internal quantum efficiency), and (2) improvement of light extraction efficiency generated in the semiconductor crystal out of the device from the semiconductor (improvement of light extraction efficiency). With respect to the latter case, only lights showing angles of not more than the critical angle at the boundary region between a semiconductor and the external environment can be taken out into the exterior, and lights showing angles of not less than the critical angle are reflected at the boundary region and returned into the interior of the semiconductor and attenuated internally, as explained by the Snell formula.
Group III-V nitride-based semiconductors have a refractive index of not less than 2, and this refractive index is different from the refractive index of a light taking out space or member (for example, atmospheric air having a refractive index of 1, epoxy encapsulating material having a refractive index of 1.5). Thus, in conventional group III-V nitride-based semiconductor light emitting devices, most of lights generated in a light emitting layer show total internal reflection angle condition, and are not taken out into the exterior, and during repetition of multireflection inside of the device, these lights are absorbed in an electrode and the like and attenuated in some cases. In the case of a device having a GaN light emitting layer encapsulated with an epoxy resin like a light emitting diode lamp, the critical angle in taking out lights from GaN to the epoxy resin would be 38° if calculated based on a refractive index 2.5 of GaN and a refractive index 1.5 of the epoxy resin. It is said that since 80% of lights are totally reflected at the boundary region between GaN and epoxy resin encapsulating material and confined in the GaN layer, only 20% of lights generated in the light emitting layer can be utilized.
Therefore, there are conventionally suggested methods for improving light extraction efficiency (JP-A Nos. 2003-258296 and 2003-218383, WO 2005/004247). JP-A Nos. 2003-258296 and 2003-218383 disclose a method in which a resin containing a block polymer or graft co-polymer and forming a micro phase separation structure in self organizing fashion is used on the surface of a GaP-based light emitting device, the micro phase separation structure of a thin film formed on the surface of the light emitting device is removed selectively, and the surface of the light emitting device is etched using residual polymer dots as an etching mask, thereby, fine roughness are formed. WO 2005/004247 discloses a method of etching the surface of an n-type layer of an GaN-based light emitting device using, as an etching mask, the residue naturally generated in dry etching.
However, there has been desired a further improvement in light extraction efficiency of a group III nitride semiconductor light emitting device, and an efficient method for producing a group III nitride semiconductor light emitting device showing high light extraction efficiency has been desired.